Yersinia pestis is the causative agent of bubonic and pneumonic plague and a potential bioweapon. Currently there is not a vaccine available to protect the public from a potential epidemic or bioterrorism event. While Y. pestis infection can be treated with antibiotics, the effective treatment window for primary pneumonic infection is very short (less than 24 hrs after exposure). Furthermore, naturally acquired resistance to antibiotics has been reported, and weaponized Y. pestis could likely be modified to be resistant to antibiotic treatment. Therefore, the development of an efficacious plague vaccine is an important scientific endeavor and public policy goal. The current lead vaccine against plague is a subunit vaccine formulation consisting of a recombinant F1-LcrV fusion protein (rF1-V) adjuvanted with alum. This vaccine has proven effective in rodent models, but has decreased efficacy in primate models and variable antibody production in clinical trials. Therefore, while rF1-V appears to be an excellent vaccine candidate for plague, it needs further improvement for a better immunogenicity profile. A series of studies have demonstrated the important and critical role of Th1 cellular responses in protection against plague. Inasmuch as alum is a Th2 adjuvant that primarily generates a robust humoral response, we hypothesized that the lead rF1-V vaccine formulation can benefit from an adjuvant system that generates a balanced mixed humoral and Th1 cellular immune responses. Towards this end, we propose the use an adjuvant platform that includes alum and the novel adjuvant SA-4-1BBL. SA-4- 1BBL is a recombinant form of costimulatory ligand 4-1BBL with demonstrated robust efficacy in generating Th1 immune responses. Preliminary data with the lead rF1-V vaccine support our hypothesis and show that the addition of SA-4-1BBL to the lead vaccine formulation generates a robust rF1-V-specific Th1 cellular immune response. In Aim 1 we will extend these preliminary studies by establishing the SA-4-1BBL dose required to produce an optimized Th1 response that complements the Th2 response elicited by alum. In Aim 2, we will directly test the hypothesis that the Th1 response elicited by SA-4-1BBL contributes to protection against plague. This will be accomplished by using a combination of immune cell depletion and mutant mouse backgrounds to define the contribution of different cell types and cytokines to vaccine-induced protection against plague. Together these studies will demonstrate that a combinatorial adjuvant platform using alum and SA-4-1BBL can produce a balanced and protective Th1/Th2 immune response. Furthermore, in addition to improving the lead candidate plague vaccine and its development for NHP and human clinical trials, the proposed adjuvant system may also be used as a platform to generate subunit vaccines and or improve the efficacy of the existing ones against a host of other intracellular pathogens.